Would you ever need to slow down a fast newt?

[ebdons]

Hi, yes maybe a silly question, but would you ever need to slow down a fast scope F4 and lower etc to improve planetary or lunar shots, or just as an object lesson, and how would you go about that apart from fitting an aperture mask or something?, but reducing the exp length to compensate for over exposure etc. maybe not a silly question? but does this make any sense to folks? ton 

[Stub Mandrel]

By adding a barlow.

I run an f8 scope at f24 for planetary.

Unless very wide, and therefore long, an F4 scope is unlikely to be good for planetary.

I also have a coma-corrector that slows my 130P-DS by a factor of 0.9 and a 0.5 (1-stop) reducer in the form of a 1.25" filter.

[vlaiv]

There are cases when you would want to slow down any scope. Primary reason for doing it would be to alter light cone from wide to narrow.

You can either do it via barlow lens - alters focal length, or aperture mask - reduces aperture but leaves focal length unchanged.

Here are couple of cases (not specific to newtonian):

Spectroscopy applications with converging cone diffraction grating for example (star analyzer) - too fast scope will create aberrations, so you can use barlow, but barlow increases focal length and this in turn makes star profiles physically larger in focal plane - maybe not a good thing because it also reduces resolution. For this particular case aperture mask is a good idea if other parameters are already good (like focal length, dispersion, pixel size ...)

H-alpha solar filters - work best at F/30 or even narrower beam (actually they very much like fully collimated beam). If you for example have refractor and daystar quark combo - you can either use telecentric lens to get to higher F/ratio - but this will prevent full disk viewing. Alternative is aperture mask - leaves focal length intact and enables full disk viewing, but still alters shape of the beam.

Achromatic refractors - aperture mask can be used to lower chromatic aberration.

With newtonian scope you can use aperture mask to lessen off axis coma - as you will effectively be using smaller section of primary - same as using smaller but slower scope (of same focal length). This does not impact planetary performance because most of the time planets are observed / imaged on axis - where coma is non existent (if scope is in good collimation) - but can help with wide field observing / imaging - it will reduce coma off axis.

 

[ebdons]

Hi Thanks, maybe I should have thought of using a tele-extender both sliding type and optical version type ? just that I use my F4 newt for everything at the moment and was seeing what I could do to keep a reasonable FOV area. Ton

[ebdons]

Hi thankyou for the info, I was using the Grating the other night and got the aberation you mention, now I know why, a lesson learned for me. clear skies my friends. Ton